It is often desirable to reinforce rubber articles by incorporating therein metal reinforcing elements. For example, tires, conveyor belts, power transmission belts, timing belts, hoses and a variety of other rubber articles are often reinforced with metal wire. In order for rubber articles which are provided with steel reinforcing elements to function effectively, it is important that good adhesion between the rubber and the metal reinforcing elements be maintained. One of the most common methods of enhancing the adhesion of wire filaments to rubber is to coat the filament with another material, for example, pneumatic vehicle tires are often reinforced with cords prepared from steel filaments which are coated with brass. Normally, steel reinforcing elements are coated with a brass that is an alloy of copper and zinc. However, ternary brass alloys that are useful for coating steel reinforcing elements are also known by those skilled in the art. For example, U.S. Pat. No. 4,347,290 discloses a ternary brass alloy which contains copper, zinc and cobalt. These and other ternary alloys, such as those containing copper, zinc and iron or tin are known to be effective in improving rubber to metal adhesion.
It is also known that various agents can be mixed into the rubber which will increase adhesion between the rubber and metal reinforcements imbedded in it. U.S. Pat. No. 3,894,903 discloses a process for improving the bonding of rubber to copper and copper alloys by incorporating into the rubber before vulcanization certain triazines, for example 2-N-hydroxyphenoxy-4-chloro-6-aminotriazine.
U.S. Pat. Nos. 3,991,130 and 3,905,947 disclose a method for improving adhesion between vulcanizable elastomeric compositions and metal surfaces by incorporating into the elastomer an organo-nickel salt and then subsequently vulcanizing the elastomeric composition while it is in contact with the metal surface.
U.S. Pat. No. 4,521,558 improves rubber adhesion to metal via the use of allyl phosphite and phosphate esters and various iminodiacetic acids and salts.
Cobalt complexes such as cobalt-mercaptobenzothiazole complexes as disclosed in UK Patent No. 914,787 have been used in natural and synthetic rubbers both as accelerators and to permit the rubber to be bonded more effectively to ferrous metals during vulcanization.
Acrylates have been added to various non-crosslinkable synthetic resin systems to improve adhesion to certain metals. For example, calcium acrylate was added to polyvinylchloride in Japan Kokai 52/3633 (Jan. 12, 1977) to improve the resin adhesion to soft steel sheet. Japan Kokai 50/77486 (Jun. 24, 1975) discloses the use of aluminum, zinc and/or calcium acrylates in non-curing PVC compositions to yield improved peel strengths with zinc wire. Polyolefin hot melt adhesives for aluminum were taught in Japanese Kokai 51/4223 (Jan. 14, 1976) to have bonding peel strength increased via the addition of zinc acrylate to the adhesive composition.
Metal diacrylates are known to function as cross-linking agents for rubbery material in the presence of various peroxide initiator systems. For example, Japanese Kokai 50/154386 (Dec. 12, 1975) discloses that adhesion to zinc-plated iron is good with EPDM/peroxide/carbon black compositions containing aluminum, zinc and/or calcium acrylate.
Similar results are taught in Japanese Patent 49/15947 (Apr. 18, 1974) which discloses a steel bonding composition containing rubber, a peroxide, carbon black and glycidyl methacrylate.
A two part liquid adhesive for steel plates is described in Japanese Kokai 58/23866 (Feb. 12, 1983) as containing an elastomer, a peroxide, an alkali metal and acrylic monomer.
Acrylates, at levels useful for promoting rubber adhesion to metal are not known to contribute to rubber crosslinking when sulfur curing systems are used.
Peroxide initiated curing systems for rubbery materials have proven to be unsatisfactory in the production of finished articles that are required to have extended flex life, for peroxide cured rubbers tend to crack when subject to repeated flexing. Additionally, peroxide curing systems tend to be expensive and because of their reactivity, they require careful handling to avoid the creation of a dangerous environment.
As a result, many industries, such as the tire and industrial belting industries prefer to use sulfur cure vulcanization systems for cross-linking rubber.